If you run out of message credits, new Subscribers will still be able to join your Lists, and receive auto-response messages. Your account will not be shut down, you won't be upgraded automatically, nor will you be billed for overage messages. However, you won't be able to send any messages out if your credit balance is at 0 credits. To send outgoing messages, you will need to either upgrade your monthly plan or purchase additional rollover credits.
You can also enable Auto Refill to prevent your account from running out of message credits.
When your message credit balance is at 0, new Subscribers will not be able to join. You will need to upgrade your plan or purchase additional message credits to continue using your account.